The present invention relates to the technical field of separating out molecules or particles by implementing separator elements generally known as xe2x80x9cmembranesxe2x80x9d and made out of inorganic materials and constituted by a rigid porous support having at least one flow channel for a fluid medium, with at least one separator layer being deposited on the surface thereof, the nature and the morphology of the layer being adapted to separate out molecules or particles contained in the fluid medium for treatment.
More precisely, the invention relates to making separator layers forming parts of inorganic membranes.
A particularly advantageous application of the invention lies in the fields of nanofiltration, ultrafiltration, microfiltration, filtration, and reverse osmosis.
In conventional manner, a membrane is defined by associating a porous support made of inorganic material, such as a ceramic, with one or more separator layers of inorganic material deposited on the surface of each flow channel and connected to one another and to the support by sintering. The function of the layers is to separate out molecular or particulate species, while the function of the support is to provide mechanical strength enabling thin layers to be made. Thus, the support provides mechanical strength without contributing to the hydraulic resistance of the membrane, while the separator layer defines permeability without contributing to mechanical strength.
In the state of the art, numerous membranes are known made from filter elements that are tubular or plane in shape. In the field of tubular membranes, the rigid porous support is elongate in shape having a right cross-section that is polygonal or circular. The porous support is arranged to include at least one channel, and preferably a series of channels that are parallel to one another and to the longitudinal axis of the porous support, each channel being in the form of a cylinder. At one end, the channels communicate with an inlet chamber for the fluid medium to be treated, and at the other end they communicate with an outlet chamber. The surface of each channel is covered in at least one separator layer serving to separate out molecules or particles contained in the fluid medium flowing along the inside of the channels in a given direction form one end of the channels to the other. By a sieving effect, such a membrane separates out molecular or particulate species from the substance to be treated, insofar as all particles or molecules of diameter larger than the pores in the membrane are stopped. During separation, fluid transfer takes place through the separator layer, and then the fluid spreads into the pores of the support and reaches the outside surface of the porous support. The portion of the fluid for treatment that passes through the separator layer and the porous support is referred to as the xe2x80x9cpermeatexe2x80x9d and it is recovered by a collector chamber surrounding the membrane.
In the technical field of plane membranes, the porous support is in the form of a block having at least one channel, and generally a series of superposed channels formed therein, each channel having a right cross-section that is polygonal, and generally rectangular. The surface of each channel is covered in at least one separator layer.
When cross-flow filtering is used, the fluid to be treated flows at high speed over the surfaces of the channels so as to generate shear stress which redisperses the matter deposited on said surfaces. Fluid friction thus arises on the surfaces of the channels and causes head loss which varies linearly as a function of channel length. This head loss depends on dimensional parameters such as the length of the membrane, its hydraulic diameter, and experimental parameters such as the flow speed, the viscosity, and the density of the fluid to be treated.
Since the force driving filtering is pressure, the pressure of the fluid to be treated is observed to decrease along the channels. Such a pressure gradient modifies the transverse flow of the permeate passing through the separator layer and then the porous body. The flow rate of the permeate thus varies along the membrane. This permeate flow rate gradient leads to non-uniformity in the separation performed by the membrane and leads to different separation conditions appearing along the channels.
In an attempt to remedy those drawbacks, U.S. Pat. No. 4,105,547 describes a cross-flow filter device implementing a system for compensating longitudinal head loss. Such a system consists in causing the permeate to flow tangentially outside the membrane in the same direction as the fluid to be treated flows tangentially inside the channels. The head loss in the permeate flow is identical to that of the fluid to be treated. Compensation thus occurs between the two head losses such that the transfer pressure is the same at all points along the channels.
Such a device presents the drawback of requiring a permeate recirculation loop to be implemented, thereby considerably complicating manufacture of such devices and increasing the energy cost associated with operating the additional loop.
To remedy those drawbacks, European patent application EP 0 870 534 proposes a macroporous support whose external porosity is modified so as to a porosity gradient to appear along the support. The porosity gradient gives rise to a permeability gradient. Because of the pressure variation, the flow of permeate passing through the membrane becomes constant. Although such a solution makes it possible to modify the support only, that technique suffers from the drawback of reducing the external porosity of the support and thus facilitates the accumulation of molecules or particles which have passed through the separator layer and which statistically can be stopped by the portion of the support having reduced porosity. In practice, the diameter of the pores on a transverse right-section of such a support increases and then diminishes at its periphery, so a risk appears of molecules or particles accumulating. Such accumulation can lead to the support being destroyed. Furthermore, the porosity is reduced solely on the outer ring of the porous support. Thus, internally the porosity of the support is not reduced. Furthermore, while separation is taking place, the pressure inside the channel decreases in the flow direction of the fluid to be treated. After passing through the separator layer, the permeate spreads out in the internal pores and flows towards the outside by seeking out zones requiring least energy. The permeate thus flows mainly via the portion of the support having the greatest porosity. Under such conditions, the porosity gradient implemented in this way leads to a permeate flow arising that is non-uniform along the length of the membrane.
The invention thus seeks to remedy the drawbacks specified above by proposing a cross-flow filter membrane adapted to obtain a permeate flow that is substantially identical along the length of the membrane and that does not present a fragile zone in which the species of the fluid to be treated accumulate because they are retained by the membrane.
To achieve this object, the membrane of the invention comprises an inorganic rigid porous support defining at least one flow channel for the fluid to be treated, the fluid flowing in a given direction, the surface of the channel being covered by at least one separator layer for separating the fluid to be treated. According to the invention, the separator layer has a thickness gradient which decreases in the flow direction of the fluid to be treated.
The invention also provides a method of making a membrane for cross-flow filtering of a fluid to be treated. According to the invention, such a method consists in covering the surface of the channel in at least one separator layer presenting a thickness gradient that decreases in the flow direction of the fluid to be treated.